1. Field of the Invention
The present invention relates generally to nuclear medical imaging devices and more particularly relates to calibration of scintillation cameras to enable correction of acquired image data for unavoidable distortions caused by the inherent physical characteristics of the detector and mask the scintillation camera.
2. Description of the Related Art
In various environments, such as in medical environments, imaging devices can include detectors that detect electromagnetic radiation emitted from radioactive isotopes or the like within a patient. The detectors typically include a sheet of scintillation crystal material that interacts with radiation, e.g., gamma rays emitted by the isotope to produce photons in the visible light spectrum known as light “events.” The scintillation camera includes one or more photodetectors such as an array of photomultiplier tubes, which detect the intensity and location of the events and accumulate this data to acquire clinically significant images that are rendered on a computer display for analysis.
Existing scintillation cameras experience spatial distortion that requires linearity correction (LC). The spatial distortion arises from the fact that the spatial coordinates of light events occurring either at the edges of or between adjacent photomultiplier tubes in a photodetector array will be computed differently than the coordinates of events occurring directly over the center of a photomultiplier tube, due to the physical limitations of the photomultiplier tube. A significant amount of effort has been made to developing correction schemes for spatial or linearity distortion (along with, e.g., the companion energy and flood corrections).
In a Gamma camera, linearity and uniformity are two product specifications that measure image quality. It is desirous to achieve specifications exceeding the National Electrical Manufactures Association (NEMA) standards and better than prior art systems and methods.
A significant effort has been allocated to correction of spatial or linearity distortion using pinhole masks. For example, see copending U.S. patent application Ser. No. 11/165,786 entitled “Peak Detection Calibration For Gamma Camera Using Non-Uniform Pinhole Aperture Grid Mask,” filed Jun. 24, 2005, assigned to the same assignee herein, the-entire contents of which are incorporated by reference; copending U.S. patent application Ser. No. 10/951,324 entitled “Imaging Devices and Methods Employing Masks with a Non-Uniform Grid of Pinhole Apertures,” filed Sep. 27, 2004, assigned to the same assignee herein, the entire contents of which are incorporated by reference; U.S. Pat. No. 6,559,450, the entire contents of which are incorporated by reference; U.S. Pat. No. 5,513,120, the entire contents of which are incorporated by reference; U.S. Patent No. 4,808,826, the entire contents of which are incorporated by reference; and U.S. Pat. No. 4,316,257, the entire contents of which are incorporated by reference.
Existing methods for the linearity correction are lengthy and generally consist of two steps: 1) Linearity Correction (LC) coefficient generation using images acquired with lead masks, and 2) LC coefficient modification using another flood by a gradient correction process to further improve the uniformity.
There are at least two problems associated with the approach. First, the gradient process improves the uniformity at the expense of the linearity. Often the better uniformity it achieves, the worse the linearity becomes. Second, the two-step process is the most time consuming procedure in the camera calibration process. Further, prior calibration methods were designed for the masks with well-separated data points along a rectangular grid and are not suitable for a new mask that has much denser population of pinhole aperture with non-uniform grid.
Therefore, there is a need for new and improved systems and methods for the correction of spatial or linearity distortion in scintillation camera.